If you are looking for the PDF, you should instead consider buying the physical book because:
The content of the letter is the highlight of the novel. It is not a typical romantic letter. It is a torn piece of paper on which a young woman has scribbled a few lines. The letter is not addressed to Kesavan Nair but is actually a response from the girl he loves, Saramma , to someone else (or perhaps a draft she discarded). Premalekhanam Malayalam Novel Pdf